Browsing by Author "Lee, Jeong-Bong"
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Item A Steep-slope Threshold Switching Selector Using Silver-doped Polycrystalline Zinc Oxide: Fabrication, Characterization, & Application for 3D X-point Memory & Neuromorphic Devices(December 2021) Sahota, Akshay; Kim, Jiyoung; Farago, Andras; Gu, Qing; Lee, Jeong-Bong; Young, Chadwin D.An assortment of emerging non-volatile memory (NVM) devices has displayed a surge of interest in being investigated for their implementation in energy-efficient bio-inspired neuromorphic computing. The intrinsic device physics of NVMs give them the capability to be employed for emulating the dynamics of a biological neuron and synapse. NVM devices are connected in a dense cross (X)-point circuit architecture thus enabling massive system-level parallelism necessary for a neural network. However, the leakage/sneak current that typically arises from neighboring unselected memory cells is considered as a stumbling block in enlarging X-point arrays. Metalfilament threshold switch has been suggested as a selector device, demonstrated on low leakage characteristics, that holds potentiality due to its straightforward metal-insulator-metal structure, superior performance, and excellent CMOS process compatibility. This dissertation demonstrates research study on the electrical and surface characterization of nano-polycrystalline silver-doped zinc oxide (ZnO) thin films for threshold switching selector device, to propose a way for amending the prevalent selector drawbacks: threshold voltage (Vth) variabilities i.e., intercell and cycle-to-cycle shifts and lousy DC cycling endurance. The current work demonstrates a novel approach to subside system variabilities by uniformly doping a crystalline selector medium i.e., ZnO with Ag metal atoms, rather than incorporating an Ag active metal layer/electrode. First, electrochemical deposition (ECD) process has been employed to slightly dope ZnO with Ag, because of its admirable dopant concentration controllability having atomic percent precision. ECD process helps in demonstrating the proof-of-concept experiment and provides an understanding of volatile switching behavior when ZnO is lightly doped with Ag. Next, “supercycle ALD” technique has been evaluated, where alternating ZnO ALD and Ag metal ALD was employed for lightly doping/delta doping ZnO with Ag. To fend off the shortcomings/drawbacks associated with both the ECD and ALD processes, RF magnetron co-sputtering process is the last fabrication method put to evaluation. Co-sputtering technique provides the wherewithal to control Ag doping levels when lightly doped composite targets (ZnO/Ag 100-x/x at. %, x=1,3,10) are employed. The switching parameters were observed to significantly improve and the trends have been explained based on surface characterizations with XPS, GIXRD, AFM, SEM, EDAX, ICP-MS, HR-TEM, and semiconductor parameter analyzer.Item A Study on Gallium Coating and Wireless Platform for Implantable Biomedical Applications(2021-04-26) Chen, Ziyu; Lee, Jeong-BongRecent research interests in wearable or implantable devices have played a significant role in advancing MEMS technologies into emerging biomedical fields. Novel materials and methods have been extensively explored in creating intrinsically flexible biomedical devices. As a novel nontoxic alternative to mercury, gallium-based liquid metals have been utilized to form functional wearable devices thanks to their unique combination of electrical and fluid properties. However, the adherent tendency of oxidized liquid metals has been a fundamental challenge that needs to be addressed in order to unleash the full potentials. This work reports gallium coating as a simple remedy to convert various microfluidic materials to nonwetting surfaces against gallium-based liquid metals. Quantitative studies on the super-lyophobicity and surface topography are presented to evaluate gallium coated surfaces as nonwetting microfluidic platform for oxidized gallium-based liquid metal droplet manipulation. Implantable functional devices, on the other hand, require wireless operation capability in order to reduce invasiveness to biological bodies while fulfilling monitoring or therapeutic functions. Intramedullary fluid modulation has been reported to enhance bone density and can be employed as a potential treatment to osteoporosis. Aiming at replacing invasive methodology used in these in vivo studies, this work presents an implantable and wirelessly operated intramedullary fluid modulator for on-demand intramedullary fluid modulation. The pressure modulation is evaluated by theoretical model as well as ex vivo and in vivo experiments. Additionally, a wireless pressure sensing system with long range transmission capability is demonstrated as a complement to wireless intramedullary fluid modulator. Details on the system design is discussed, and evaluation results in terms of pressure response and transmission range is presented.Item Advancements in Hardware/firmware and Applications for CCi-MOBILE: a Cochlear Implant Research Platform(2021-12-01T06:00:00.000Z) Ghosh, Ria; Hansen, John H. L.; Thuraisingham, Bhavani; Lee, Jeong-Bong; Nourani, Mehrdad; Basu, Kanad; Goldsworthy, Raymond LHearing impairment is a pervasive problem which occurs due to the detrimental damage caused to the inner ear. Assistive Hearing Devices such as Cochlear Implants (CIs) and Hearing Aids (HAs) are designed to restore hearing, personalize rehabilitation, and enrich the listening experience. Although signal processing and machine learning research has greatly improved audio processing, the rigid design requirements of commercial CI sound processors make it difficult to explore novel algorithms for research investigations and conduct longitudinal studies. This thesis presents the design, development, clinical evaluation, and applications of CCi-MOBILE, a computationally powerful signal processing testing platform built specifically for researchers in the CI/HA field along with implementing multiple additional features to the platform. This custom-made, portable research platform allows researchers to design and perform complex speech processing algorithm assessment offline and in real-time through user-friendly, software-mediated open-source tools with implants manufactured by Cochlear Corporation. The design includes a lightweight custom circuit board comprising of an on-board FPGA to be used in conjunction with a computing platform such as a PC/tablet/laptop/smartphone based on the requirement of CI/HA signal processing algorithms. The processing pipeline for CI and HA stimulation is discussed followed by results from an acute study with implant users’ speech intelligibility in quiet and noisy conditions. The platform supports testing of algorithms for unilateral, bilateral, and bimodal hearing impairment. A major obstruction to accurate source localization for bimodal and bilateral CI users is the distortion of interaural time and level difference cues (ITD and ILD), and limited ITD sensitivity. Various CI research interfaces developed by either academic or industry sponsored research teams support proposed signal processing and psychoacoustic investigations but have limited ability to efficiently validate bimodal and/or bilateral algorithms. To overcome such challenges; verification, and validation of the synchronized bilateral (electric-electric) and bimodal (electric-acoustic) outputs is performed, in an authenticated and efficient way, to support localization algorithmic and experimental investigations. It has been hypothesized that variable stimulation rate for exciting the electrode array can aid for better speech perception and increased spectral information. Hence, a new multi-rate implant strategy including time-varying stimulation rates has been proposed in this work. Lastly, expanding the capabilities of the platform to ensure long-term sustainability, a real-time data streaming link between the platform and a cloud-based data repository is established to enable remote-test facilities along with an algorithm implementation and testing in naturalistic environments. We discuss implementation feasibility, and hypothesized performance of these approaches individually, and collectively, on the perceptual benefit for researchers working towards the welfare of the hearing-impaired community.Item An Efficient 3D Mathematical Model to Predict Structural Dynamics and Chatter in Cold Rolling Mills(2022-05-01T05:00:00.000Z) Patel, Akash; Lee, Jeong-Bong; Malik, Arif; Griffith, D. Todd; Park, Wooram; Qian, Dong; Zipf, MarkThe research described in this dissertation aims to provide a highly efficient predictive computational tool to improve the dimensional quality of cold rolled metal strip, particularly for high-value, thin specialty alloys. The corresponding objectives of this work are to (1) understand the transfer of high-fidelity roll grinding errors that may generate complex geometric defects on the strip, and to investigate a novel method for correction of such defects; and (2) develop a highly efficient 3D dynamic predictive model for the time-history of mill and strip transient behavior, including highly damaging chatter vibrations. First, a novel approach that can potentially correct for high-fidelity geometric defects in cold rolled strip is proposed. High- fidelity flatness defects in thin cold-rolled strip that arise from highly localized thickness strain variations present an ongoing challenge to the metals industry. A primary cause of such defects, based on rolling practice, but for which the effects have not been rigorously investigated, may be the transfer of localized diameter deviations from the work rolls that arise from roll grinding errors due to grinding performance inaccuracies. The proposed research addresses the effects of high-fidelity roll diameter deviation transfer to the strip, as well as their correction. Parametric case studies are first undertaken using a 4-high mill to investigate the influences that roll diameter, strip reduction, strip width, and material strength have on the 3D transfer of high- fidelity work roll diameter deviations to the rolled sheet. The studies are conducted with an efficient 3D mathematical roll-stack model that predicts the associated high-fidelity strip thickness profile deviations using the simplified-mixed finite element method (SM-FEM). Reduction deviations, which strongly correlate to strip flatness/shape defects, are first quantified and analyzed to understand the transfer characteristics of localized work-roll grinding deviations relative to benchmarked perfectly smooth work rolls. Results of the study reveal that the high- fidelity transfer depends not only on the specific roll grinding deviation amplitude and mill loading, but also on the location of the roll diameter deviations along the roll face length due to non-negligible 3D bulk roll-stack deformations, as well as the effective stiffness ratio between the work roll and the strip. The inability of conventional flatness control devices to correct for high-fidelity roll diameter deviations is also demonstrated. Based on this work, suggested is a novel corrective approach to identify customized work roll grinding profiles that are tailored to strip with specific pre-existing high-fidelity defect patterns generated in previous rolling passes. Using the described SM-FEM modeling technique, high-fidelity “corrective” roll diameter profiles could eventually be applied in-situ during rolling, whereby the profiles are “engineered” to account for the predicted 3D mill deflections, contact force distributions, and coupled micro/macro scale deformation mechanics. Following investigation of the SM-FEM formulation to high-fidelity static problems involving the transfer of roll grinding error to the strip, the SM- FEM method is adapted to create a general purpose 3D structural dynamics model capable of predicting the transient behavior of the mill components and strip thickness profile geometry. Over the last three decades, computational models have been developed and employed in effort to understand dynamic disturbances in the rolling operation. Such disturbances can potentially lead to self-excitation (chatter vibrations) and result in significant gauge variations in the exit strip, as well as strip rupture and/or damage to the mill in extreme cases. Numerous challenges exist, however, in adequately modeling the 3D dynamic behavior. For instance, highly coupled relationships exist between several rolling process parameters, including the rolling force/torque, strip entry/exit tensions, rolling speed, roll gap profile, friction, neutral point, etc. In addition, both “hard” and “soft” nonlinearities are present, including continuous changes in the roll/strip contact conditions, elastic-plastic deformation of the strip, and nonlinear elastic flattening the rolls. These factors make it very difficult to effectively and efficiently model the rolling operation even under a quasi-static (or steady-state) assumption. Moreover, the existing models that account for structural dynamics of the rolling process exploit many simplifications, such as modeling the mill structure as linear lumped parameter system, and symmetry in the motions of rolls, among other assumptions. Even with these assumptions, the current state-of-the-art models are generally not capable of accommodating conventional strip thickness profile/flatness control mechanisms, such as roll bending, roll shifting, or non-uniform machined roll profiles, which severely restricts the flexibility/applicability to accommodate complex mill structures. A 3D general purpose dynamic model that can incorporate profile/flatness control mechanisms in addition to complex mill configurations (like 12-high or 20-high cluster mills) can provide significant new insights into rolling dynamics and chatter investigations. Accordingly, the presented research combines the static SM-FEM mathematical formulation with a Newmark- Beta time integration technique to develop a highly-efficient, stable, high-fidelity global-stiffness based transient structural dynamics model. Case studies are carried out to demonstrate the features and capabilities of the presented model in addressing the aforementioned research gaps and challenges. Based on the results, the presented structural dynamics model is able to efficiently capture time histories due to discrete disturbances on both vertical and cluster-type mill configurations. For chatter investigation, however, an appropriate roll-bite model to capture the relationships among the coupled rolling process parameters that influence the roll-bite contact mechanics is required to be coupled to accommodate the real time interactions between the structural dynamics and roll-bite mechanics. In addition to the aforementioned assumptions and simplifications in mill structural dynamic modeling, presence of both hard, and soft non- linearities as well as material non-linearities in the roll bite contact mechanics has led to exclusive use of linearized relationship in the current state-of-the-art chatter models, while 3D chatter models are non-existent in the literature. Accordingly, following the development of general-purpose 3D dynamic model, the dynamic simplified mixed-finite element method (D- SM-FEM) is coupled with a roll-bite process model. A critical part in replicating the dynamic interaction between the rolling process and the mill structural dynamics in this work relates to real-time variations in the “working point” or “operating point” relationship between specific rolling force and the plastic strain of the rolled strip which changes according to perturbations in the roll gap, position of entry/exit plane, entry/exit velocity, and tensions. These variations in the working point are incorporated in the presented chatter model via the concept of a “dynamic strip modulus” based on the secant (or tangent) relationship between the specific rolling force and plastic strain at the working point, but where the strip modulus is updated at every time-step. Case studies are presented using 4-high mill with aim to demonstrate the ability of the presented 3D chatter model to address the lack of available chatter models in literature employing 3D bulk body deformation effects, and to address some of the limitations identified above. The capabilities of the model to predict the stability, or dynamic instability is also illustrated with accompanying 3D plots showing the true mode shapes. Case studies are also undertaken to demonstrate the effect of asymmetric (with varying lower housing stiffness) mill stand assumptions. The results reveal interesting phase relationships not elucidated in previous research, as well as detailed effects from the 3D modeling on the strip profile and shape/flatness.Item Fabrication of a Microneedle/CNT Hierarchical Micro/Nano Surface Electrochemical Sensor and its In-Vitro Glucose Sensing CharacterizationYoon, Youngsam; Lee, Gil Sik; Yoo, Koangki; Lee, Jeong-Bong; 0000 0001 3865 4673 (Lee, GS)We report fabrication of a microneedle-based three-electrode integrated electrochemical sensor and in-vitro characterization of this sensor for glucose sensing applications. A piece of silicon was sequentially dry and wet etched to form a 15 x 15 array of tall (approximately 380 μm) sharp silicon microneedles. Iron catalyst was deposited through a SU-8 shadow mask to form the working electrode and counter electrode. A multi-walled carbon nanotube forest was grown directly on the silicon microneedle array and platinum nano-particles were electrodeposited. Silver was deposited on the Si microneedle array through another shadow mask and chlorinated to form a Ag/AgCl reference electrode. The 3-electrode electrochemical sensor was tested for various glucose concentrations in the range of 3~20 mM in 0.01 M phosphate buffered saline (PBS) solution. The sensor's amperometric response to the glucose concentration is linear and its sensitivity was found to be 17.73 ± 3 μA/mM-cm². This microneedle-based sensor has a potential to be used for painless diabetes testing applications.;Item Liquid Metal Actuation-Based Reversible Frequency Tunable Monopole Antenna(Amer Inst Physics) Kim, Daeyoung; Pierce, Richard G.; Henderson, Rashaunda; Doo, Seok Joo; Yoo, Koangki; Lee, Jeong-BongWe report the fabrication and characterization of a reversible resonant frequency tunable antenna based on liquid metal actuation. The antenna is composed of a coplanar waveguide fed monopole stub printed on a copper-clad substrate, and a tunnel-shaped microfluidic channel linked to the printed metal. The gallium-based liquid metal can be injected and withdrawn from the channel in response to an applied air pressure. The gallium-based liquid metal is treated with hydrochloric acid to eliminate the oxide layer, and associated wetting/sticking problems, that arise from exposure to an ambient air environment. Elimination of the oxide layer allows for reliable actuation and repeatable and reversible tuning. By controlling the liquid metal slug on-demand with air pressure, the liquid metal can be readily controllable to connect/disconnect to the monopole antenna so that the physical length of the antenna reversibly tunes. The corresponding reversible resonant frequency changes from 4.9 GHz to 1.1 GHz. The antenna properties based on the liquid metal actuation were characterized by measuring the reflection coefficient and agreed well with simulation results. Additionally, the corresponding time-lapse images of controlling liquid metal in the channel were studied.Item One-Step Combined-Nanolithography-And-Photolithography for a 2d Photonic Crystal TM Polarizer(MDPI AG) Choi, Kyung-Hak; Huh, J.; Cui, Yonghao; Trivedi, Krutarth; Hu, Walter; Ju, B. -K; Lee, Jeong-BongPhotonic crystals have been widely investigated since they have great potential to manipulate the flow of light in an ultra-compact-scale and enable numerous innovative applications. 2D slab photonic crystals for the telecommunication C band at around 1550 nm have multi-scale structures that are typically micron-scale waveguides and deep sub-micron-scale air hole arrays. Several steps of nanolithography and photolithography are usually used for the fabrication of multi-scale photonic crystals. In this work, we report a one-step lithography process to pattern both micron and deep sub-micron features simultaneously for the 2D slab photonic crystal using combined-nanoimprint-andphotolithography. As a demonstrator, a 2D silicon photonic crystal transverse magnetic (TM) polarizer was fabricated, and the operation was successfully demonstrated.Item PDMS Based Coplanar Microfluidic Channels for the Surface Reduction of Oxidized GalinstanLi, G.; Parmar, M.; Kim, Daeyoung; Lee, Jeong-Bong; Lee, D. -WGalinstan has the potential to replace mercury-one of the most popular liquid metals. However, the easy oxidation of Galinstan restricts wide applicability of the material. In this paper, we report an effective reduction method for the oxidized Galinstan using gas permeable PDMS (polydimethlysiloxane)-based microfluidic channel. The complete study is divided into two parts-reduction of Galinstan oxide and behavior of reduced Galinstan oxide in a microfluidic channel. The reduction of Galinstan oxide is discussed on the basis of static as well as dynamic angles. The contact angle analyses help to find the extent of reduction by wetting characteristics of the oxide, to optimize PDMS thickness and to select suitable hydrochloric acid (HCl) concentration. The highest advancing angle of 155° and receding angle of 136° is achieved with 200 μm thick PDMS film and 37 wt% (weight percent) HCl solution. The behavior of reduced Galinstan oxide is analyzed in PDMS-based coplanar microfluidic channels fabricated using a simple micromolding technique. Galinstan in the microfluidic channel is surrounded by another coplanar channel filled with HCl solution. Due to the excellent permeability of PDMS, HCl permeates through the PDMS wall between the two channels (interchannel PDMS wall) and achieves a continuous chemical reaction with oxidized Galinstan. A Lab VIEW controlled syringe pump is used for observing the behavior of HCl treated Galinstan in the microfluidic channel. Further optimization of the microfluidic device has been conducted to minimize the reoxidation of reduced Galinstan oxide in the microfluidic channel.Item Perovskite Nanophotonic Devices and Topological Photonic Devices(2021-05-01T05:00:00.000Z) Li, Zhitong; Gu, Qing; Minary, Majid; Zhang, Chuanwei; Lee, Jeong-Bong; Zakhidov, Anvar A.Solution processed organic-inorganic lead halide perovskites have rapidly emerged as a promising gain material for development of the next generation of nanophotonic device ranging from nanolasers, nano LEDs, and solar cells. Here, continuous-wave operation of MAPbI3 perovskite nanolaser is achieved at room temperature with ultralow threshold, which is enabled by thermal nanoimprint lithography that directly patterns perovskite into laser cavities and improves perovskite’s emission characteristics. In the meantime, hyperbolic metamaterials and metasurfaces (HMMs), a special class of anisotropic media, has drawn tremendous research attention recently owing to its remarkable ability to manipulate electromagnetic waves at the subwavelength scale. However, the inevitable metal loss hinders the development of HMMs. Here, a luminescent perovskite HMM operating at 760 nm is achieved using alternating layers of MAPbI3 perovskite and Au, where the loss in Au is maximally compensated by MAPbI3. Simultaneously, topological photonics is a rapidly emerging field, aiming to apply topological physics in photonic systems. The topological protected photonic edge mode is immune to the system disorders and imperfections. However, all photonic edge modes reported in the pioneering works are from lattice systems. Here, a topological band theory is developed in continuous HMM through a nonHermitian Hamiltonian formulated Maxwell’s equations. Two types of edge mode can be induced by including gyromagnetic and chiral effect in HMM and can be numerically observed. Finally, a topological micro ring laser array that possesses edge mode lasing is designed and experimentally achieved on the III-V semiconductor platform.Item Processing and Characterization of CeRAM: a Non-volatile Non-filamentary Resistive Memory(May 2023) Prasad, Rohan 1999-; Young, Chadwin D.; Frensley, William R.; Lee, Jeong-BongMemory technologies have been evolving for a long time to provide durable and fast operation while not being very expensive. SRAM, DRAM, and Flash are traditional memories with advantages over each other in terms of speed, cost, reliability, and non-volatility. In addition, several emerging memory technologies are coming forward to solve one or more problems associated with existing memory technologies. CeRAM is one such memory technology projected to have several benefits over existing memory technologies. CeRAM, where ‘Ce’ stands for ‘Correlated Electrons’, is a resistive memory that undergoes switching through orbital interaction of atoms and bandgap variation in the material. In addition to being non-volatile, CeRAM is seen to have fast switching, and due to a rather simple fabrication process, CeRAM is relatively less expensive, as well. This gives CeRAM a potential edge over the existing memory technologies in terms of speed, cost, and memory retention. This project explores the operation of CeRAM memory devices and how durable and reliable they can be. The thesis indulges in the fabrication methodology of the device and investigates the performance through different tests. Stable two-state operation is demonstrated in these memory devices in terms of setting and resetting. Moreover, these devices offer promising endurance from room temperature to high-temperature environments (i.e., up to 200C), thereby expanding the scope of application of these memory devices. This project attempts to establish a functioning memory device that can work well in terms of writing or programming the memory, reading the distinctive memory states, competent endurance, and high-temperature operation. The results are promising, and more work can enhance the performance of these devices. It can potentially lead to reliable non-volatile memory technology that does not compromise speed and cost-effectiveness.Item Studies on Improving Quality Factor of 2D GMR Gratings and on Nonwetting Properties of Plasma-treated Polymer Surfaces Towards Liquid Metal Microfluidics(2021-04-23) Babu, Sachin; Lee, Jeong-BongPolydimethylsiloxane (PDMS) is a biocompatible elastomer that is used widely in microfabrication, and this work presents three studies that utilize this elastomer in the areas of strain sensors, thin film coatings, and liquid metal microfluidics. The first study is on improving the quality factor of a 2D guided-mode resonance (GMR) strain sensor, which is a binary grating made of PDMS and titanium dioxide. To improve the quality factor, a slotting design rule is developed that can be applied to any grating design. To study the effect of the slotting design rule, finite element analysis simulations were performed, and the results indicate that the design rule helps produce resonances with at least a 6-fold increase in quality factor over the original design as well as more axially-symmetric sensitivities. The second study concerns the CF4/O2 plasma-treatment of polymers (PDMS being one of several studied) which creates a nonwetting surface toward gallium-based liquid metals. Gallium-based liquid metals tend to wet a variety of materials, and a method that allows conversion of a previously wetting polymer surface to a nonwetting one can help open new areas of research for liquid metal applications. The study conducts a variety of surface-level analyses – contact angle goniometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and apparent surface free energy analysis – to show that the cause of the nonwetting property is primarily due to surface roughness. The third study is on the feasibility of CF4/O2 plasma-treated PDMS channels to allow actuation and generation of surface-oxidized gallium based liquid metal (oxLM) droplets. The results of the study indicate that actuation and generation of oxLM droplets is not feasible due to the surface oxide of the liquid metal.